BPJS Ketenagakerjaan is the law of the public who formed by government to implement the social assurance program according the act No 24 2011. The purpose of this research is to find out the activity of marketing division to introduce BPJS Ketenagakerjaan in a formal sector to increase the number of participants companies around Semarang, and also to know the obstacle that happened in the implement that activity. The techniques of the analysis is descriptive qualitative, and the method of collecting data use literature review, observation, and interview. To increase the participants of companies, BPJS Ketenagakerjaan using marketing mix strategy. The result of this research, there are seven marketing mix strategy (7P) applied to improve the number of participants, that is product, price, promotion, place, participants, process and physical evidence. The application of marketing mix at BPJS Ketenagakerjaan is considered very effective to increase the number of the company register as participant of BPJS Ketenagakerjaan. It was proven with increasing data about the number of the company register from 2014 until 2015. The obstacles at BPJS Ketenagakerjaan is there are many companies and workers who that have not been registered due to lack of policy detail membership information. BPJS Ketenagakerjaan need to improve the socialization.